VSL-DSP Open Source Driver

Open source Linux kernel module for the PreSonus AudioBox VSL family of USB audio interfaces.

The module detects any member of the AudioBox VSL family that is plugged into the USB bus and logs the connection and disconnection to the kernel ring buffer. It is a passive detector: it never claims the audio interface, so the standard snd-usb-audio driver keeps owning the device and ALSA playback, capture, and mixer continue to work without re-plug. The driver is licensed under GPL-2.0-or-later.

Supported hardware

Model	USB ID (vendor:product)	Bus
AudioBox 22 VSL	194f:0101	USB 2.0
AudioBox 44 VSL	194f:0102	USB 2.0
AudioBox 1818 VSL	194f:0103	USB 2.0

The three models share the same UAC2 audio interface and the same HID control plane. Adding support for a new product ID is a one-line change in audiobox_vsl.h.

Features

• Automatic detection of every AudioBox VSL model via USB VID/PID.
• Coexists with snd-usb-audio (no interface claim, no audio loss).
• Connection and disconnection logged to the kernel ring buffer.
• Optional auto-load at boot via /etc/modules-load.d/.
• Single source of truth in audiobox_vsl.h for every supported PID.
• CMocka unit test suite for the model lookup table and accessor.
• Hardening flags on every userspace test compile (-Wall -Wextra -Werror -fstack-protector-strong -D_FORTIFY_SOURCE=2).
• AddressSanitizer + UndefinedBehaviorSanitizer target (make asan).
• GPL-2.0-or-later.

Requirements

• Linux kernel headers for the running kernel.
• GCC and GNU make.
• For the test suite: libcmocka-dev (Debian/Ubuntu/Kali).
• Root access to install and load the module.

Quick start

1. Install build dependencies (one time, as root)

sudo ./install.sh

This installs build-essential, perl, linux-headers-$(uname -r), and (if available) libcmocka-dev.

2. Verify the build environment

./configure

3. Build the module and run the test suite

make

This produces audiobox_vsl.ko in the project directory and runs the CMocka unit tests for the model lookup table.

4. Install the module

sudo make install

This copies the module to /lib/modules/$(uname -r)/extra/, runs depmod, and (in a follow-up step you choose to enable) can register it in /etc/modules-load.d/ for automatic loading at boot.

5. Load the module

sudo make modprobe

or:

sudo modprobe audiobox_vsl

6. Verify detection

Plug in any AudioBox VSL device and watch the kernel log:

dmesg --follow | grep audiobox_vsl

Expected output:

audiobox_vsl: detected 'AudioBox 22 VSL' (194f:0101)
audiobox_vsl:   manufacturer=PreSonus Audio Electronics product=AudioBox 22 VSL serial=...
audiobox_vsl: disconnected 'AudioBox 22 VSL' (194f:0101)

The model name reflects the actual product ID of the plugged-in device.

7. Uninstall

sudo make uninstall

Test targets

make test    # build and run the CMocka unit test suite
make asan    # build and run the test suite under ASan+UBSan
make info    # print resolved build variables
make deb     # build Debian package (.deb) for the kernel module
make help    # list every available target

The test suite covers the model lookup table and accessor: audiobox_lookup_model(pid) returns a pointer to the matching model entry for every supported PID and NULL otherwise. The suite also asserts that the model table is well formed: PIDs are unique, product names are non-empty, and the count matches the array size.

Adding a new product ID

To support another model that uses the same UAC2 control plane:

1. Add an enumerator to audiobox_model_pid_t in audiobox_vsl.h (for example AUDIOBOX_MODEL_XX_VSL = 0x0104U).
2. Append a matching entry to audiobox_models[] in the same file with the canonical product name.
3. The kernel device table in audiobox_vsl.c is generated from that single array; no other source change is required.
4. Add a unit test in tests/test_audiobox_vsl.c that asserts the new PID resolves to the new product name.

Repository layout

.
├── audiobox_vsl.c                 kernel detector module
├── audiobox_vsl.h                 public interface and configuration
├── Makefile                       single source of truth for build, test, install
├── install.sh                     installs build dependencies (root only)
├── configure                      verifies the build environment
├── README.md                      this file
├── CLAUDE.md                      working contract for AI agents and maintainers
├── LICENSE                        GPL-2.0-or-later
├── spec/
│   └── audiobox_vsl.md            BDD specification for the detector
├── src/
│   ├── vsl_dsp_logic.c            userspace DSP math library
│   ├── vsl_dsp_logic.h
│   ├── vsl_dsp_transport.c        userspace HID transport
│   └── vsl_dsp_transport.h
├── tests/
│   └── test_audiobox_vsl.c        CMocka unit test suite
├── docs/                          auxiliary documentation
├── .github/                       issue and pull request templates
└── legacy/                        historical artefacts (Python PoC, DKMS
                                   installer, packet captures); see
                                   legacy/README.md

How it works

audiobox_vsl.c registers a struct usb_driver whose device table contains one entry per supported product ID. The table is built from audiobox_models[] in audiobox_vsl.h, which is the only place where the product IDs are listed.

When the USB core matches a device, probe is called. The handler:

1. Looks up the product ID with audiobox_lookup_model().
2. Logs the model name, the raw VID/PID, and the USB string descriptors (manufacturer, product, serial) to the kernel ring buffer via dev_info.
3. Returns -ENODEV so the USB core does not bind the driver to the interface. The standard snd-usb-audio driver therefore remains the owner and ALSA continues to provide full audio functionality (playback, capture, mixer) without any re-plug.

When the device is unplugged, disconnect logs the model name and the VID/PID and returns.

The detector never touches the HID control endpoint, never allocates memory in the hot path, and never formats untrusted input. The canonical product name comes from a static const table; the USB string descriptors are logged verbatim by the kernel dev_info API using %s with a NULL guard.

License

GPL-3.0-or-later. See the LICENSE file for the full text.